Copolymer for liquid crystal alignment agent, liquid crystal alignment agent including the same, and liquid crystal alignment film and liquid crystal display device using the same

ABSTRACT

A polymer having excellent liquid crystal alignment and electrical properties and thus is suitable for use as a liquid crystal alignment agent, a liquid crystal alignment agent including the same, a liquid crystal alignment film formed from the liquid crystal alignment agent, and a liquid crystal display device including the liquid crystal alignment film are provided.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a 35 U.S.C. 371 National Phase Entry Applicationfrom PCT/KR2018/011168, filed Sep. 20, 2018, which claims the benefit ofpriority from Korean Patent Application No. 10-2017-0128186 filed onSep. 29, 2017 with the Korean Intellectual Property Office, thedisclosures of which are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The present invention relates to a copolymer having excellent liquidcrystal alignment and electrical properties and thus is suitable for useas a liquid crystal alignment agent, a liquid crystal alignment agentincluding the same, a liquid crystal alignment film formed from theliquid crystal alignment agent, and a liquid crystal display deviceincluding the liquid crystal alignment film.

BACKGROUND ART

In order to obtain uniform brightness and a high contrast ratio in aliquid crystal display device, it is essential for the liquid crystalsto be uniformly aligned. The liquid crystal alignment agent serves as adirector in the arrangement of liquid crystal molecules, and thus, whenthe liquid crystals move by an electric field to form an image, it helpsthem take an appropriate direction.

Polyimide, polyamide, polyester, and the like are widely known asconventional liquid crystal alignment agents. Among them, particularly,polyimide is excellent in heat resistance, affinity with liquid crystal,mechanical strength, etc., and therefore is used for many liquid crystaldisplay devices.

However, in recent years, as the demand for a lower power display hasincreased, it has been found that the liquid crystal alignment agent canaffect not only the basic properties such as the alignment property ofthe liquid crystal but also the electrical properties such as anafterimage generated by a direct current/alternating voltage, and avoltage holding ratio. Thus, there is a growing need for the developmentof a liquid crystal alignment material capable of simultaneouslyrealizing excellent liquid crystal alignment and electrical properties.

For this purpose, various attempts have been made to change thestructure itself of the liquid crystal alignment agent, through a methodof changing monomers used for the production of the liquid crystalalignment agent or by combining a plurality of different monomers,thereby improving the physical/chemical properties thereof. However,these attempts have not yet reached a dramatic improvement in physicalproperties.

Therefore, there is a need to develop a novel liquid crystal alignmentagent having excellent liquid crystal alignment and electricalproperties.

DETAILED DESCRIPTION OF THE INVENTION Technical Problem

An object of the present invention is to provide a copolymer havingexcellent liquid crystal alignment and electrical properties and thus issuitable for use as a liquid crystal alignment agent.

Another object of the present invention is to provide a liquid crystalalignment agent, a liquid crystal alignment film, and a liquid crystaldisplay device using the above-described copolymer for a liquid crystalalignment agent.

Technical Solution

The present invention provides a copolymer for a liquid crystalalignment agent including one or more repeating units selected from thegroup consisting of a repeating unit represented by the followingChemical Formula 1, a repeating unit represented by the followingChemical Formula 2, and a repeating unit represented by the followingChemical Formula 3; and one or more repeating units selected from thegroup consisting of a repeating unit represented by the followingChemical Formula 4, a repeating unit represented by the followingChemical Formula 5, and a repeating unit represented by the followingChemical Formula 6:

wherein, in Chemical Formulae 1 to 6,

R¹, R², R³, and R⁴ are each independently hydrogen or a C₁₋₁₀ alkyl,provided that R¹ and R² are not both hydrogen, and R³ and R⁴ are notboth hydrogen,

X¹, X³, and X⁵ are each independently a tetravalent organic grouprepresented by the following Chemical Formula 7:

wherein, in Chemical Formula 7,

R⁵, R⁶, R⁷, and R⁸ are each independently hydrogen or a C₁₋₆ alkyl,

X², X⁴, X⁶, X⁷, X⁸, and X⁹ are each independently a tetravalent organicgroup derived from a hydrocarbon having 4 to 20 carbon atoms or atetravalent organic group, wherein in the above tetravalent organicgroup, one or more of H is substituted with a halogen or one or more of—CH₂— is substituted with —O—, —CO—, —S—, —SO—, —SO₂—, or —CONH— toprevent direct binding with oxygen or sulfur atoms,

in Chemical Formulae 1 to 3,

Y¹, Y², Y³, Y⁴, Y⁵, and Y⁶ are each independently a divalent organicgroup represented by the following Chemical Formula 8:

wherein, in Chemical Formula 8,

R⁹ and R¹³ are each independently a halogen, a cyano, a C₁₋₁₀ alkyl, aC₂₋₁₀ alkenyl, a C₁₋₁₀ alkoxy, a C₁₋₁₀ fluoroalkyl, or a C₁₋₁₀fluoroalkoxy,

p and q are each independently an integer of 0 to 4,

L¹ is a single bond, —O——, —CO—, —S—, —SO₂—, —C(CH₃)₂—, —C(CF₃)₂—,—CONH—, —COO—, —(CH₂)_(z)—, —O(CH₂)_(z)O—, —O(CH₂)_(z)—,—OCH₂—C(CH₃)₂—CH₂O—, —COO—(CH₂)_(z)—OCO—, or —OCO—(CH₂)_(z)—COO—,wherein each z is independently an integer of 1 to 10,

k and m are each independently an integer of 0 to 3, and

n is an integer of 0 to 3,

in Chemical Formulae 4 to 6,

Z¹, Z², and Z³ are each independently a divalent organic grouprepresented by the following Chemical Formula 9:

wherein, in Chemical Formula 9,

A¹ is an element of Group 15,

R¹¹ is hydrogen or a C₁₋₁₀ alkyl,

a is an integer of 1 to 3, and

A², A³, A⁴, and A⁵ are nitrogen or carbon, provided that at least one ofA² to A⁵ is nitrogen and the rest are carbon.

Hereinafter, a copolymer for a liquid crystal alignment agent accordingto a specific embodiment of the present invention, a preparation methodthereof, a liquid crystal alignment agent including the copolymer, amethod of producing a liquid crystal alignment film using the liquidcrystal alignment agent, and a liquid crystal display device includingthe liquid crystal alignment film thus produced will be described inmore detail.

Throughout the specification, when one part “includes” one constituentelement, unless otherwise specifically described, this does not meanthat another constituent element is excluded, but means that anotherconstituent element may be further included.

As used herein, the term “substituted” means that a hydrogen atom in acompound is changed to another substituent, and a position to besubstituted is not limited as long as the position is one at which thehydrogen atom is substituted, that is, a position at which thesubstituent may be substituted, and when two or more are substituted,the two or more substituents may be the same as or different from eachother.

As used herein, the term “substituted or unsubstituted” means thatsubstitution is performed by one or more substituent groups selectedfrom the group consisting of deuterium; a halogen group; a cyano group;a nitro group; a hydroxyl group; a carbonyl group; an ester group; animide group; an amide group; an amino group; a carboxy group; a sulfonicacid group; a sulfonamide group; a phosphine oxide group; an alkoxygroup; an aryloxy group; an alkylthioxy group; an arylthioxy group; analkylsulfoxy group; an arylsulfoxy group; a silyl group; a boron group;an alkyl group; a cycloalkyl group; an alkenyl group; an aryl group; anaralkyl group; an aralkenyl group; an alkylaryl group; an arylphosphinegroup, or a heterocyclic group containing at least one of N, O, and Satoms, or there is no substituent group, or substitution is performed bya substituent group where two or more substituent groups of theexemplified substituent groups are linked, or there is no substituentgroup. For example, the term “substituent group where two or moresubstituent groups are linked” may refer to a biphenyl group. That is,the biphenyl group may be an aryl group, or may be interpreted as asubstituent group where two phenyl groups are connected.

In the present specification,

means a bond connected to another substituent group, and a direct bondmeans a case where another atom does not exist in a portion representedby L.

In the present specification, the alkyl group may be straight-chained orbranched, and the number of carbon atoms thereof is not particularlylimited, but is preferably 1 to 10. According to another embodiment, thealkyl group has 1 to 6 carbon atoms. Specific examples of the alkylgroup include methyl, ethyl, propyl, n-propyl, isopropyl, butyl,n-butyl, isobutyl, tert-butyl, sec-butyl, 1-methyl-butyl, 1-ethyl-butyl,pentyl, n-pentyl, isopentyl, neopentyl, tert-pentyl, hexyl, n-hexyl,1-methylpentyl, 2-methylpentyl, 4-methyl-2-pentyl, 3,3-dimethylbutyl,2-ethylbutyl, heptyl, n-heptyl, 1-methylhexyl, cyclopentylmethyl,cycloheptylmethyl, octyl, n-octyl, tert-octyl, 1-methylheptyl,2-ethylhexyl, 2-propylpentyl, n-nonyl, 2,2-dimethylheptyl,1-ethyl-propyl, 1,1-dimethyl-propyl, isohexyl, 2-methylpentyl,4-methylhexyl, 5-methylhexyl, and the like, but are not limited thereto.

The fluoroalkyl group having 1 to 10 carbon atoms may be one in which atleast one hydrogen atom in an alkyl group having 1 to 10 carbon atoms issubstituted with fluorine, and the fluoroalkoxy group having 1 to 10carbon atoms may be one in which at least one hydrogen atom in an alkoxygroup having 1 to 10 carbon atoms is substituted with fluorine.

The halogen may be fluorine (F), chlorine (Cl), bromine (Br), or iodine(I).

The element of Group 15 may be nitrogen (N), phosphorus (P), arsenic(As), antimony (Sb), or bismuth (Bi).

The nitrogen oxide is a compound in which a nitrogen atom and an oxygenatom are bonded, and the nitrogen oxide functional group means afunctional group containing a nitrogen oxide in the functional group.Examples of the nitrogen oxide functional group include a nitro group(—NO₂) and the like.

A copolymer for a liquid crystal alignment agent according to thepresent invention is characterized by including at least one of therepeating units of Chemical Formulae 1 to 3 prepared from a reactionproduct containing an imide-containing diamine compound having aspecific structure along with at least one of the repeating units ofChemical Formulae 4 to 6 prepared from a reaction product containing anitrogen atom-containing diamine compound having a specific structure.

When polyimide is conventionally used as a liquid crystal alignmentfilm, a polyimide precursor, polyamic acid, or polyamic acid esterhaving excellent solubility is coated and dried to form a coating film,and then converted to polyimide through a high-temperature heattreatment, which is then subjected to light irradiation for alignmenttreatment. However, a lot of light irradiation energy is required toobtain sufficient liquid crystal alignment by light irradiation of thepolyimide film, and an additional heat treatment is also required tosecure alignment stability after light irradiation. Use of a lot oflight irradiation energy and additional high-temperature heat treatmentare very disadvantageous in terms of process cost and time, andtherefore there is a limitation in applying it to practicalmass-production.

Accordingly, the present inventors found that when at least one of therepeating units of Chemical Formulae 1 to 3 prepared from a reactionproduct containing an imide-containing diamine compound having aspecific structure are included in the copolymer for a liquid crystalalignment agent, imide repeating units that has been already imidizedare present, and thus anisotropy is directly generated by lightirradiation without the high-temperature heat treatment after formationof the coating film, and subsequently, heat treatment is performed tocomplete an alignment film. Accordingly, light irradiation energy may begreatly reduced, and a liquid crystal alignment film having an enhancedalignment property and stability may be produced even by a simpleprocess including a single heat treatment process.

Further, the present inventors found that when at least one of therepeating units of Chemical Formulae 4 to 6 prepared from a reactionproduct containing a nitrogen atom-containing diamine compound having aspecific structure, along with at least one of the repeating units ofChemical Formulae 1 to 3, are included in the copolymer for a liquidcrystal alignment agent, a liquid crystal alignment film produced fromthe copolymer may have a high voltage holding ratio even at a hightemperature, and a reduction in contrast ratio or an afterimagephenomenon may be improved, and alignment stability due to heat stressand mechanical strength of the alignment film may be improved, therebycompleting the present invention.

According to one embodiment of the present invention, provided is acopolymer for a liquid crystal alignment agent including one or morerepeating units selected from the group consisting of the repeating unitrepresented by Chemical Formula 1, the repeating unit represented byChemical Formula 2, and the repeating unit represented by ChemicalFormula 3; and one or more repeating units selected from the groupconsisting of the repeating unit represented by Chemical Formula 4, therepeating unit represented by Chemical Formula 5, and the repeating unitrepresented by Chemical Formula 6.

The copolymer for a liquid crystal alignment agent may include a blockcopolymer, a random copolymer, an alternating copolymer, a graftcopolymer, or the like.

Specifically, the polymer according to one embodiment may include therepeating units of Chemical Formulae 1 to 6 wherein X¹, X³, and X⁵ areeach independently a tetravalent organic group represented by ChemicalFormula 7, X², X⁴, X⁶, X⁷, X^(8,) and X⁹ are each independently atetravalent organic group derived from a hydrocarbon having 4 to 20carbon atoms or a tetravalent organic group, wherein in the abovetetravalent organic group, one or more of H is substituted with ahalogen or one or more of —CH₂— is substituted with —O—, —CO—, —S—,—SO—, —SO₂—, or —CONH— to prevent direct binding with oxygen or sulfuratoms.

For example, X², X⁴, X⁶, X⁷, X⁸, and X⁹ may each independently be atetravalent organic group represented by the following Chemical Formula10:

wherein, in Chemical Formula 10, R⁵, R⁶, R⁷, and R⁸ are eachindependently hydrogen or a C₁₋₆ alkyl, R¹² and R¹³ are eachindependently hydrogen or a C₁₋₁₀ alkyl, and L² is any one selected fromthe group consisting of a single bond, —O—, —CO—, —S—, —SO—, —SO₂—,—CR¹⁴R¹⁵—, —CONH—, —COO—, —(CH₂)_(b)—, —O(CH₂)_(b)O—,—COO—(CH₂)_(b)—OCO—, —HN—(CH₂)_(b)—NH—, —R¹⁴N—(CH₂)_(b)—NR¹⁵—,phenylene, and combinations thereof, wherein R¹⁴ and R¹⁵ are eachindependently hydrogen, a C₁₋₁₀ alkyl, or a C₁₋₁₀ fluoroalkyl, and eachb is independently an integer of 1 to 10.

Further, the polymer according to one embodiment may include therepeating units of Chemical Formulae 1 to 3, wherein Y¹, Y², Y³, Y⁵, andY⁶ may each independently be a divalent organic group represented byChemical Formula 8.

In Chemical Formula 8, hydrogen is bound to a carbon which is notsubstituted with R⁹ or R¹⁰, and when p or q is an integer of 2 to 4, aplurality of R⁹ or R¹⁰ may be the same or different substituents.Further, in Chemical Formula 8, k and m may each independently be aninteger of 0 to 3, or 1 to 3, and n may be an integer of 0 to 3, or 0 or1.

Chemical Formula 8 corresponds to a part of the repeating unit derivedfrom the imide-containing diamine having a specific structure which is aprecursor used in the formation of the copolymer for a liquid crystalalignment agent.

More specifically, Chemical Formula 8 may be the following ChemicalFormula 11 or Chemical Formula 12:

wherein, in Chemical Formula 12, L³ is a single bond, —O—, —SO₂—, or—CR¹⁶R¹⁷—, wherein R¹⁶ and R¹⁷ are each independently hydrogen or aC₁₋₁₀ alkyl.

Preferably, Chemical Formula 11 may be the following Chemical Formula11-1:

Further, the Chemical Formula 12 may be the following Chemical Formula12-1:

wherein, in Chemical Formula 12-1, L³ is O or CH₂.

Further, the polymer according to one embodiment may have repeatingunits of Chemical Formulae 4 to 6, wherein Z¹, Z², and Z³ may eachindependently be a divalent organic group represented by ChemicalFormula 9. The Z¹, Z², and Z³ may be defined as the divalent organicgroup represented by Chemical Formula 4 to provide a copolymer for aliquid crystal alignment agent having various structures, which mayexhibit the above-descried effects.

In Chemical Formula 9, A¹ may be an element of Group 15, and the elementof Groups 15 may be nitrogen (N), phosphorus (P), arsenic (As), antimony(Sb), or bismuth (Bi). The R¹¹ is a functional group binding to A¹, andmay bind to the A¹ element by a number represented by a. Preferably, inChemical Formula 9, A¹ may be nitrogen, R¹¹ may be hydrogen, and a maybe 1.

On the other hand, by satisfying the condition that in Chemical Formula9 at least one of A² to A⁵ is nitrogen and the rest are carbon, ChemicalFormula 9 may form an asymmetric structure which does not form symmetrywith respect to the center point or the center line due to the nitrogenatom. Chemical Formula 9 is a repeating unit derived from a nitrogenatom-containing diamine having a specific structure, which is aprecursor used for the formation of a copolymer for a liquid crystalalignment agent, and this is considered to be due to the use of anasymmetric diamine as described later.

The functional group represented by Chemical Formula 9 has a structuralfeature in which two aromatic cyclic compounds, preferably aheteroaromatic cyclic compound and an aromatic cyclic compound, arebound via a secondary amine group or a tertiary amine group. Therefore,the liquid crystal alignment agent may satisfy an equivalent level ormore of alignment property or afterimage property, and may have animproved voltage holding ratio, thereby realizing excellent electricalproperties.

On the other hand, when two aromatic cyclic compounds are bound througha single bond without a secondary amine group or a tertiary amine group,there are technical problems that the alignment property of the liquidcrystal alignment agent becomes poor, and the voltage holding ratio isrelatively reduced.

Further, in the case where neither of the two aromatic cyclic compoundsbound through a secondary amine group or a tertiary amine group containa nitrogen atom, even if the imidization reaction of polyamic acid orpolyamic acid ester formed by the reaction of amine and acid anhydrideproceeds (e.g., via 230° C. heat treatment), a sufficient imidizationreaction does not proceed, and thus there is a limitation that animidization rate decreases within the final liquid crystal alignmentfilm.

Further, the functional group represented by Chemical Formula 9 ischaracterized in that only the amine group and hydrogen are bound toeach of two aromatic cyclic compounds, preferably the heteroaromaticcyclic compound and the aromatic cyclic compound, and other substituentsare not introduced. When a substituent such as a fluoroalkyl group isintroduced into the heteroaromatic cyclic compound or the aromaticcyclic compound, there is a technical problem that the alignmentproperty deteriorates due to the substituent.

More specifically, in Chemical Formula 9, one of A² to A⁵ may benitrogen and the rest may be carbon. In Chemical Formula 4, one of A²and A⁵ is nitrogen and the rest are carbon, and A³ and A⁴ may be carbon.That is, the aromatic ring containing A² to A⁵ in Chemical Formula 9 mayhave a pyridine structure. Accordingly, the liquid crystal displaydevice to which the copolymer for a liquid crystal alignment agent ofone embodiment is applied may realize a high voltage holding ratio andliquid crystal alignment property.

Further, Chemical Formula 9 may include one or more repeating unitsselected from the group consisting of the following Chemical Formula9-1, Chemical

Formula 9-2, and Chemical Formula 9-3:

wherein, in Chemical Formula 9-1, Chemical Formula 9-2, and ChemicalFormula 9-3, descriptions of A¹, A², A³, A⁴, A⁵, R¹¹ and a include theabove description of Chemical Formula 9.

As described above, as the repeating unit of Chemical Formula 9 includesone or more functional groups selected from the group consisting ofChemical Formulas 9-1, Chemical Formulas 9-2, and Chemical Formulas 9-3,an excellent liquid crystal alignment property may be realized.

On the other hand, in the field of copolymers for liquid crystalalignment agents conventionally known in the art, from the viewpoint ofnot recognizing the constitution of the asymmetric diamine or therepeating unit derived therefrom, along with the above-describedimide-containing diamine having the specific structure, and effectsresulting therefrom at all, the copolymer including the repeating unitsof Chemical Formula 1 to 3 along with the repeating units of ChemicalFormulae 4 to 6 is considered to be novel. Accordingly, an excellentcoating property may be exhibited, thereby providing a copolymer for aliquid crystal alignment agent which may implement a high imidizationrate while having excellent processing properties, and may haveexcellent electrical properties such as an afterimage generated by thedirect current/alternating voltage, and the voltage holding ratio.

In particular, the copolymer for a liquid crystal alignment agent of oneembodiment may include one or more repeating units selected from thegroup consisting of repeating units represented by the followingChemical Formulae 13 to 21:

wherein, in Chemical Formulas 13 to 21, descriptions of R¹ to R⁴, X¹ toX⁹, Y¹ to Y⁶, and Z¹ to Z³ include the above descriptions of ChemicalFormulae 1 to 6. Further, each of m¹ to m¹⁸ is a number of thecorresponding repeating units, and each is independently an integer of 1to 500. In this regard, each of m¹ to m¹⁸ may be adjusted according to acomposition ratio of the two kinds of diamine compounds, that is, mol %of the imide-containing diamine having the specific structure and theasymmetric diamine.

In this regard, a molar ratio between repeating units represented by oneor more repeating units selected from the group consisting of therepeating unit represented by Chemical Formula 1, the repeating unitrepresented by Chemical Formula 2, and the repeating unit represented byChemical Formula 3, and one or more repeating unit selected from thegroup consisting of the repeating unit represented by Chemical Formula4, the repeating unit represented by Chemical Formula 5, and therepeating unit represented by Chemical Formula 6, may be 1:99 to 99:1,40:60 to 95:5, or 50:50 to 90:10.

For example, when the molar ratio of m², m⁴, m⁶, m⁸, m¹⁰, m¹², m¹⁴, m¹⁶,or m¹⁸ which is the number of the repeating unit derived from asymmetricdiamine is regarded as m^(even), m^(even) may be 1 to 99 mol %, 5 to 60mol %, or 10 to 50 mol %, and accordingly, when the molar ratio of m¹,m³, m⁵, m⁷, m⁹, m¹¹, m¹³, m¹⁵, or m¹⁷ which is the number of therepeating unit derived from the imide-containing diamine having thespecific structure is regarded as m^(odd), m^(odd) may be 1-m^(even),that is, 99 to 1 mol %, 95 to 40 mol %, or 90 to 50 mol %.

In particular, when the copolymer for a liquid crystal alignment agentof the present invention includes the imidized imide repeating unit andthe repeating unit derived from the asymmetric diamine in the abovemolar ratio, excellent photoreactive property and liquid crystalalignment property which are obtained from the imide repeating unit andthe excellent electrical properties obtained from the repeating unitderived from the asymmetric diamine may complement each other, andtherefore, excellent coating properties may be exhibited, therebyproducing a liquid crystal alignment film which may implement a highimidization rate while having excellent processing properties, and mayhave excellent electrical properties such as an afterimage generated bythe direct current/alternating voltage, and the voltage holding ratio,and a liquid crystal alignment film which may have an excellentalignment property and electrical properties at the same time.

On the other hand, among the repeating units represented by ChemicalFormulae 1 to 6, the polymer according to one embodiment may include therepeating units represented by Chemical Formulae 1 to 4 which arederived from the imide-containing diamine having the specific structure,in an amount of 1 mol % to 99 mol %, preferably 20 mol % to 95 mol %,based on the total repeating units.

As described above, when the copolymer including a specific amount ofthe imide repeating units represented by Chemical Formulae 1 to 4 isused, the copolymer includes a predetermined amount of already imidizedimide repeating units, and thus a liquid crystal alignment film havingan excellent alignment property and stability can be produced even whenthe high-temperature heat treatment is omitted and light is directlyirradiated.

If the repeating units represented by Chemical Formulae 1 to 4 areincluded at less than the above-mentioned content range, sufficientalignment properties may not be exhibited and alignment stability may bedeteriorated. If the content of the repeating units represented byChemical Formulae 1 to 4 exceeds the above-mentioned content range,there is a problem that the solubility is lowered and thus it isdifficult to prepare a stable alignment solution capable of coating.Accordingly, it is preferable to include the repeating units representedby Chemical Formulae 1 to 4 within the above-mentioned content range,because it can provide a copolymer for a liquid crystal alignment agenthaving excellence in all of storage stability, electrical properties,alignment properties, and alignment stability.

Further, the repeating units represented by Chemical Formulae 5 and 6may be included in an appropriate amount depending on the desiredproperties.

For example, the repeating unit represented by Chemical Formula 5 may beincluded in an amount of 0.1 mol % to 90 mol %, preferably 5 mol % to 80mol %, based on the total repeating units represented by ChemicalFormulae 1 to 4. When the repeating unit represented by Chemical Formula5 is included in an amount of less than 0.1 mol %, excellent electricalproperties may not be exhibited. When the repeating unit represented byChemical Formula 5 is included in an amount of more than 90 mol %,alignment stability may be deteriorated. Therefore, the repeating unitrepresented by Chemical Formula 5 exhibits appropriate solubility withinthe above-mentioned range and thus may provide a copolymer for a liquidcrystal alignment agent which may implement a high imidization ratewhile having excellent processing properties.

For example, the repeating unit represented by Chemical Formula 6 may beincluded in an amount of 0.1 mol % to 90 mol %, preferably 5 mol % to 80mol %, based on the total repeating units represented by ChemicalFormulae 1 to 4. Within such a range, excellent coating properties maybe exhibited, thereby providing a copolymer for a liquid crystalalignment agent which may implement a high imidization rate while havingexcellent processing properties.

As described above, the copolymer for a liquid crystal alignment agentof the present invention may include a predetermined amount of alreadyimidized imide repeating units, and thus anisotropy is directlygenerated by light irradiation without the high-temperature heattreatment after formation of the coating film, and subsequently, heattreatment is performed to complete an alignment film. In addition, thecopolymer for a liquid crystal alignment agent of the present inventionmay include the repeating units derived from a nitrogen atom-containingdiamine compound having a specific asymmetric structure, and thus a highvoltage holding ratio may be obtained even at a high temperature, and areduction in contrast ratio or an afterimage phenomenon may be improved,thereby improving electrical properties.

Meanwhile, the copolymer for a liquid crystal alignment agent may have aweight average molecular weight of 1000 g/mol to 200,000 g/mol. Theweight average molecular weight means a weight average molecular weightin terms of polystyrene measured by a gel permeation chromatography(GPC) method. In the process of determining the weight average molecularweight in terms of polystyrene measured by the GPC method, a commonlyknown analyzing device, a detector such as a refractive index detectoror a UV-detector, and an analytical column may be used. Commonly appliedconditions of temperature, solvent, and flow rate may be used. Specificexamples of the measurement conditions may include a temperature of 40°C., a mixed solvent of dimethylformamide (DMF)/tetrahydrofuran (THF),and a flow rate of 0.5 mL/min to 1.0 mL/min.

Such a polymer may be used as a liquid crystal alignment agent toprovide a liquid crystal alignment film which realizes excellentstability and reliability.

Examples of the method of producing the copolymer for a liquid crystalalignment agent are not particularly limited. For example, a method ofproducing the copolymer for a liquid crystal alignment agent includingthe steps of: reacting a mixture including diamine of the followingChemical Formula 22 and diamine of the following Chemical Formula 23with tetracarboxylic acid or an anhydride thereof; and imidizing thereaction product with tetracarboxylic acid or an anhydride thereof maybe used:

wherein, in Chemical Formula 22,

A¹ is an element of Group 15, R¹¹ is hydrogen or C₁₋₁₀ alkyl, a is aninteger of 1 to 3, and A², A³, A⁴, and A⁵ are nitrogen or carbon,provided that at least one of A² to A⁵ is nitrogen, and the rest arecarbon,

wherein, in Chemical Formula 23,

R⁵, R⁶, R⁷, and R⁸ are each independently hydrogen or a C₁₋₆ alkyl, andY′ and Y″ are each independently a divalent organic group represented bythe following Chemical Formula 24,

wherein, in Chemical Formula 24,

R⁹ and R¹⁰ are each independently a halogen, a cyano, a C₁₋₁₀ alkyl, aC₂₋₁₀ alkenyl, a C₁₋₁₀ alkoxy, a C₁₋₁₀ fluoroalkyl, or a C₁₋₁₀fluoroalkoxy,

p and q are each independently an integer of 0 to 4,

L¹ is a single bond, —O—, —CO—, —S—, —SO₂—, —C(CH₃)₂—, —C(CF₃)₂—,—CONH—, —COO—, —(CH₂)_(z)—, —O(CH₂)_(z)O—, —O(CH₂)_(z)—, —OCH₂—C(CH₃)₂—CH₂O—, —COO—(CH₂)_(z)—OCO—, or —OCO—(CH₂)_(z)—COO—, wherein each z isindependently an integer of 1 to 10,

k and m are each independently an integer of 0 to 3, and

n is an integer of 0 to 3 or an integer of 3.

Preferably, in Chemical Formula 22, A¹ may be a nitrogen atom, one of A²and A⁵ may be nitrogen and the rest may be carbon, and A³ and A⁴ may becarbon.

Specific descriptions of the substituents in Chemical Formula 22,Chemical Formula 23, and Chemical Formula 24 are the same as thosedescribed in Chemical Formulae 1 to 6.

Here, the diamine of Chemical Formula 22 and the diamine of ChemicalFormula 23 may be mixed at a molar ratio of 1:99 to 99:1, 40:60 to 95:5,or 50:50 to 90:10.

The mixture including the diamine of Chemical Formula 22 and the diamineof Chemical Formula 23 may be reacted with tetracarboxylic acid or ananhydride thereof commonly used for the preparation of polyamic acids,for example, pyromellitic dianhydride, 3,3′,4,4′-biphenyltetracarboxylic acid dianhydride, or 1,2,4,5-cyclohexanetetracarboxylicdianhydride, to prepare a copolymer composed of polyamic acid, polyamicacid ester, or a mixture thereof.

Alternatively, if necessary, in addition to the diamines of ChemicalFormula 22 and Chemical Formula 23, various types of diamine compounds,which are widely known in the field generally related to liquid crystalalignment agents, for example, p-phenylenediamine, 4,4-oxydianiline,4,4′-methylenedianiline, or the like, may be mixed to prepare polyamicacid, polyamic acid ester, or a mixture thereof.

The reaction conditions may be appropriately adjusted with reference tothe production conditions of polyamic acid known in the technical fieldto which the present invention belongs. Then, the obtained polyamicacid, polyamic acid ester, or a mixture thereof may be imidized toprepare a copolymer for a liquid crystal alignment agent having theabove-mentioned repeating units of Chemical Formulae 1 to 3 and therepeating units of Chemical Formulae 4 to 6 at the same time.

On the other hand, according to another embodiment of the invention, aliquid crystal alignment agent including the copolymer is provided.

Since the liquid crystal alignment agent includes the above-mentionedcopolymer, it may secure an excellent alignment property without aninitial thermosetting process, effectively suppress deterioration ofstability and reliability due to the decomposition reaction of thepolymer during the baking process and storage, and exhibit excellentcoating properties and simultaneously exhibit an excellent imideconversion ratio.

Such a liquid crystal alignment agent may be provided through a varietyof methods known in the technical field to which the present inventionbelongs, except that it includes the above-mentioned copolymer.

For a non-limiting example, the above-mentioned copolymer may bedissolved or dispersed in an organic solvent to provide a liquid crystalalignment agent.

Specific examples of the organic solvent include N,N-dimethylformamide,N,N-dimethylacetamide, N-methyl-2-pyrrolidone, N-methylcaprolactam,2-pyrrolidone, N-ethylpyrrolidone, N-vinylpyrrolidone, dimethylsulfoxide, tetramethylurea, pyridine, dimethyl sulfone, hexamethylsulfoxide, y-butyrolactone, 3-methoxy-N,N-dimethylpropanamide,3-ethoxy-N, N-di methyl propanam ide,3-butoxy-N,N-1,3-dimethyl-imidazolidinone, ethyl amyl ketone, methylnonyl ketone, methyl ethyl ketone, methyl isoamyl ketone, methylisopropyl ketone, cyclohexanone, ethylene carbonate, propylenecarbonate, diglyme, 4-hydroxy-4-methyl-2-pentanone, and the like. Thesesolvents may be used alone or in a mixture thereof.

In addition, the liquid crystal alignment agent may further includeother components in addition to the copolymer and the organic solvent.For a non-limiting example, when the liquid crystal alignment agent hasbeen coated, an additive capable of improving the uniformity of filmthickness or the surface smoothness, improving adhesion between theliquid crystal alignment film and the substrate, changing the dielectricconstant and conductivity of a liquid crystal alignment film, orincreasing the denseness of the liquid crystal alignment film, may befurther included. Such additive may be exemplified by a variety ofsolvents, surfactants, silane-based compounds, dielectric substances,crosslinkable compounds, etc.

On the other hand, according to still another embodiment of theinvention, a method of producing a liquid crystal alignment film usingthe liquid crystal alignment agent as described above is provided. Themethod of producing a liquid crystal alignment film may include thesteps of: coating the liquid crystal alignment agent onto a substrate toform a coating film (step 1); drying the coating film (step 2);irradiating the coating film with light or rubbing the coating filmimmediately after the drying step to perform an alignment treatment(step 3); and heat-treating and curing the alignment-treated coatingfilm (step 4).

The step 1 is a step of coating the above-described liquid crystalalignment agent onto a substrate to form a coating film.

The method of coating the liquid crystal alignment agent onto asubstrate is not particularly limited, and for example, a method such asscreen printing, offset printing, flexographic printing, inkjet, and thelike may be used.

Furthermore, the liquid crystal alignment agent may be those which aredissolved or dispersed in an organic solvent. Specific examples of theorganic solvent include N,N-dimethylformamide, N,N-dimethylacetamide,N-methyl-2-pyrrolidone, N-methylcaprolactam, 2-pyrrolidone,N-ethylpyrrolidone, N-vinylpyrrolidone, dimethyl sulfoxide,tetramethylurea, pyridine, dimethyl sulfone, hexamethyl sulfoxide,y-butyrolactone, 3-methoxy-N,N-dimethylpropanamide,3-ethoxy-N,N-dimethylpropanamide, 3-butoxy-N,N-dimethylpropanamide,1,3-dimethyl-imidazolidinone, ethyl amyl ketone, methyl nonyl ketone,methyl ethyl ketone, methyl isoamyl ketone, methyl isopropyl ketone,cyclohexanone, ethylene carbonate, propylene carbonate, diglyme,4-hydroxy-4-methyl-2-pentanone, ethylene glycol monomethyl ether,ethylene glycol monomethyl ether acetate, ethylene glycol monoethylether, ethylene glycol monoethyl ether acetate, ethylene glycolmonopropyl ether, ethylene glycol monopropyl ether acetate, ethyleneglycol monoisopropyl ether, ethylene glycol monoisopropyl ether acetate,ethylene glycol monobutyl ether, ethylene glycol monobutyl etheracetate, and the like. These solvents may be used alone or in a mixturethereof.

In addition, the liquid crystal alignment agent may further includeother components in addition to the organic solvent. For a non-limitingexample, when the liquid crystal alignment agent has been coated, anadditive capable of improving the uniformity of film thickness orsurface smoothness, improving adhesion between the liquid crystalalignment film and the substrate, changing the dielectric constant andconductivity of a liquid crystal alignment film, or increasing thedenseness of the liquid crystal alignment film, may be further included.Such an additive may be exemplified by a variety of solvents,surfactants, silane-based compounds, dielectric substances,crosslinkable compounds, etc.

The step 2 is a step of drying the coating film which is formed bycoating the liquid crystal alignment agent onto a substrate.

In the step of drying the coating film, a method such as heating of acoating film or vacuum evaporation may be used, and the drying may bepreferably carried out at 50° C. to 150° C., or at 60° C. to 140° C.

The step 3 is a step of irradiating the coating film with light orrubbing the coating film immediately after the drying step to performalignment treatment.

In the present disclosure, the “coating film immediately after thedrying step” means that light is directly irradiated, after the dryingstep, without carrying out heat treatment at a temperature equal to orhigher than that of the drying step, and steps other than the heattreatment may be added.

More specifically, when a liquid crystal alignment film is preparedusing a conventional liquid crystal alignment agent including polyamicacid or polyamic acid ester, it includes a step of irradiating lightafter essentially performing a high-temperature heat treatment forimidization of polyamic acid. However, when a liquid crystal alignmentfilm is prepared using the liquid crystal alignment agent of oneembodiment described above, it does not include the heat treatment step,but light is directly irradiated to perform alignment treatment, andthen the alignment-treated coating film is cured by heat treatment,thereby preparing a liquid crystal alignment film.

In the alignment treatment step, the light irradiation is performed byirradiating polarized ultraviolet rays having a wavelength of 150 nm to450 nm. In this case, the intensity of the light exposure may varydepending on the kind of the copolymer for a liquid crystal alignmentagent, and preferably, energy of 10 mJ/cm² to 10 J/cm², and morepreferably energy of 30 mJ/cm² to 2 J/cm², may be irradiated.

As for the ultraviolet rays, polarized ultraviolet rays selected amongultraviolet rays subjected to polarization treatment by a method ofpassing through or reflecting with a polarizing device using a substratein which a dielectric anisotropic material is coated on the surface of atransparent substrate such as quartz glass, soda lime glass, sodalime-free glass, etc., a polarizer plate on which aluminum or metalwires are finely deposited, or a Brewster's polarizing device by thereflection of quartz glass, etc., are irradiated to perform thealignment treatment. Herein, the polarized ultraviolet rays may beirradiated perpendicularly to the surface of the substrate, or may beirradiated by directing at an angle of incidence toward a specificangle. By this method, the alignment ability of the liquid crystalmolecules is imparted to the coating film.

Further, in the alignment treatment step, a rubbing treatment may use amethod of using a rubbing cloth. More specifically, in the rubbingtreatment, the surface of the coating film after the heat treatment stepmay be rubbed in one direction while rotating a rubbing roller of whicha rubbing cloth is attached to a metal roller.

The step 4 is a step of heat-treating and curing the alignment-treatedcoating film.

The step of heat-treating and curing the alignment-treated coating filmis a step that is carried out after the irradiation of light even in themethod of preparing a liquid crystal alignment film using a copolymerfor a liquid crystal alignment agent including polyamic acid or polyamicacid ester in the past, and is distinguished from the step of heattreatment performed for imidizing the liquid crystal alignment agentbefore irradiating light or while irradiating light, after coating theliquid crystal alignment agent onto a substrate.

Herein, the heat treatment may be carried out by a heating means such asa hot plate, a hot air circulation path, an infrared ray furnace, andthe like, and the heat treatment is preferably carried out at atemperature of 150° C. to 300° C., or 180° C. to 250° C.

On the other hand, after a step of drying the coating film (step 2), astep of heat-treating the coating film immediately after the drying stepat a temperature equal to or higher than that of the drying step may befurther included, if necessary. The heat treatment may be performed by aheating means such as a hot plate, a hot air circulation path, aninfrared furnace, or the like, and is preferably performed at 150° C. to250° C. In this process, the liquid crystal alignment agent may beimidized.

That is, the method of producing a liquid crystal alignment film mayinclude the steps of: coating the above-mentioned liquid crystalalignment agent onto a substrate to form a coating film (step 1); dryingthe coating film (step 2); heat-treating the coating film immediatelyafter the drying step at a temperature equal to or higher than that ofthe drying step (step 3); irradiating the heat-treated coating film withlight or rubbing the coating film to perform alignment treatment (step4); and heat-treating and curing the alignment-treated coating film(step 5).

According to still another embodiment of the invention, a liquid crystalalignment film produced by the above-described method is provided.

As described above, when the liquid crystal alignment agent composed ofthe copolymer including one or more repeating units selected from thegroup consisting of the repeating units represented by Chemical Formula1, Chemical Formula 2, and Chemical Formula 3 along with one or morerepeating units selected from the group consisting of the repeatingunits represented by Chemical Formula 4, Chemical Formula 5, andChemical Formula 6 is used, it is possible to produce a liquid crystalalignment film having enhanced durability according to the improvementof film strength.

According to still another embodiment of the present invention, a liquidcrystal display device including the liquid crystal alignment filmdescribed above is provided.

The liquid crystal alignment film may be introduced into a liquidcrystal cell by a known method, and likewise, the liquid crystal cellmay be introduced into a liquid crystal display device by a knownmethod. The liquid crystal alignment film may be produced from thecopolymer including one or more of the repeating units represented byChemical Formulae 1 to 3 and one or more of the repeating unitsrepresented by Chemical Formulae 4 to 6, and thus may implementexcellent stability together with excellent physical properties.Specifically, it is possible to provide a liquid crystal display devicehaving excellent electrical properties due to a high voltage holdingratio at a high temperature and a low frequency, having a reduction inthe deterioration of the contrast ratio or in an image sticking(afterimage) phenomenon, and also having excellent film strength.

Advantageous Effects

According to the present invention, a copolymer for a liquid crystalalignment agent having excellent liquid crystal alignment and electricalproperties, and a preparation method thereof, may be provided.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, the present invention will be described in more detail inthe following examples. However, these examples are for illustrativepurposes only, and the scope of the present invention is not intended tobe limited by the following examples.

Preparation Examples 1˜2: Preparation of Diamine Preparation Example 1

After 18.3 g (100 mmol) of 2-chloro-5-nitropyridine (compound 1) and12.5 g (98.6 mmol) of para-phenylenediamine (p-PDA, compound 2) werecompletely dissolved in about 200 mL of dimethyl sulfoxide (DMSO), 23.4g (200 mmol) of trimethylamine (TEA) was added thereto, and the mixturewas stirred at room temperature for 12 hours. When the reaction wascompleted, the reaction product was charged into a container containingabout 500 mL of water and stirred for about 1 hour. A solid obtained byfiltration thereof was washed with about 200 mL of water and about 200mL of ethanol to synthesize 16 g (61.3 mmol) of a compound 3 (yield:60%).

The compound 3 was dissolved in about 200 mL of a 1:1 mixed solution ofethyl acetate (EA) and THF, 0.8 g of palladium (Pd)/carbon (C) was addedthereto, and the mixture was stirred for about 12 hours under a hydrogenatmosphere. After completion of the reaction, the reaction mixture wasfiltered through a pad of Celite and then concentrated to obtain 11 g ofa diamine compound 4 (pIDA) (yield: 89%).

Preparation Example 2

1,3-dimethylcyclobutane-1,2,3,4-tetracarboxylic acid dianhydride(DMCBDA, compound 5) and 4-nitroaniline were dissolved indimethylformamide (DMF) to prepare a mixture. Subsequently, the mixturewas allowed to react at about 80° C. for about 12 hours to preparepolyamic acid of compound 6. Next, the polyamic acid was dissolved inDMF, and acetic acid anhydride and sodium acetate were added to preparea mixture. Then, polyamic acid of compound 6 included in the mixture wasimidized at about 90° C. for about 4 hours to obtain a compound 7. Theimide of compound 7 thus obtained was dissolved in dimethylacetamide(DMAc), and then Pd/C was added to prepare a mixture. The mixture wasreduced at about 45° C. under a hydrogen atmosphere of about 6 bar for20 hours to prepare a diamine compound 8 (DMICPD).

<Synthesis Examples 1˜8 and Comparative Synthesis Examples 1˜6:Synthesis of Polymer for Liquid Crystal Alignment Agent> SynthesisExample 1

1.921 g (0.010 mol) of the diamine (pIDA) prepared in PreparationExample 1 and 34.918 g (0.086 mol) of the diamine (DMICPD) prepared inPreparation Example 2 were completely dissolved in 322.091 g ofanhydrous N-methyl pyrrolidone (NMP).

Then, under an ice bath, 20.0 g (0.089 mol) of1,3-dimethylcyclobutane-1,2,3,4-tetracarboxylic acid dianhydride(DMCBDA) was added to the solution and stirred at room temperature forabout 16 hours to prepare a copolymer P-1 for a liquid crystal alignmentagent. The molecular weight of the copolymer was confirmed by GPC, andas a result, the weight average molecular weight (Mw) was 23,000 g/mol.

Synthesis Example 2

7.204 g (0.036 mol) of the diamine (pIDA) prepared in PreparationExample 1 and 14.549 g (0.036 mol) of the diamine (DMICPD) prepared inPreparation Example 2 were completely dissolved in 208.269 g ofanhydrous N-methyl pyrrolidone (NMP).

Then, under an ice bath, 15.0 g (0.067 mol) of1,3-dimethylcyclobutane-1,2,3,4-tetracarboxylic acid dianhydride(DMCBDA) was added to the solution and stirred at room temperature forabout 16 hours to prepare a copolymer P-2 for a liquid crystal alignmentagent. The molecular weight of the copolymer was confirmed by GPC, andas a result, the weight average molecular weight (Mw) was 25,500 g/mol.

Synthesis Example 3

1.974 g (0.010 mol) of the diamine (pIDA) prepared in PreparationExample 1 and 35.887 g (0.089 mol) of the diamine (DMICPD) prepared inPreparation Example 2 were completely dissolved in 327.881 g ofanhydrous N-methyl pyrrolidone (NMP).

Then, under an ice bath, 20.0 g (0.092 mol) of pyromellitic dianhydride(PMDA) was added to the solution and stirred at room temperature forabout 16 hours to prepare a copolymer P-3 for a liquid crystal alignmentagent. The molecular weight of the copolymer was confirmed by GPC, andas a result, the weight average molecular weight (Mw) was 24,000 g/mol.

Synthesis Example 4

7.404 g (0.037 mol) of the diamine (pIDA) prepared in PreparationExample 1 and 14.953 g (0.037 mol) of the diamine (DMICPD) prepared inPreparation Example 2 were completely dissolved in 211.688 g ofanhydrous N-methyl pyrrolidone (NMP).

Then, under an ice bath, 15.0 g (0.069 mol) of pyromellitic dianhydride(PMDA) was added to the solution and stirred at room temperature forabout 16 hours to prepare a copolymer P-4 for a liquid crystal alignmentagent. The molecular weight of the copolymer was confirmed by GPC, andas a result, the weight average molecular weight (Mw) was 23,500 g/mol.

Synthesis Example 5

1.464 g (0.007 mol) of the diamine (pIDA) prepared in PreparationExample 1 and 26.605 g (0.066 mol) of the diamine (DMICPD) prepared inPreparation Example 2 were completely dissolved in 272.388 g ofanhydrous N-methyl pyrrolidone (NMP).

Then, under an ice bath, 20.0 g (0.068 mol) of biphenyl tetracarboxylicdianhydride (BPDA) was added to the solution and stirred at roomtemperature for about 16 hours to prepare a copolymer P-5 for a liquidcrystal alignment agent. The molecular weight of the copolymer wasconfirmed by GPC, and as a result, the weight average molecular weight(Mw) was 26,500 g/mol.

Synthesis Example 6

7.318 g (0.037 mol) of the diamine (pIDA) prepared in PreparationExample 1 and 14.780 g (0.037 mol) of the diamine (DMICPD) prepared inPreparation Example 2 were completely dissolved in 238.56 g of anhydrousN-methyl pyrrolidone (NMP).

Then, under an ice bath, 20.0 g (0.068 mol) of biphenyl tetracarboxylicdianhydride (BPDA) was added to the solution and stirred at roomtemperature for about 16 hours to prepare a copolymer P-6 for a liquidcrystal alignment agent. The molecular weight of the copolymer wasconfirmed by GPC, and as a result, the weight average molecular weight(Mw) was 25,000 g/mol.

Synthesis Example 7

1.921 g (0.010 mol) of the diamine (pIDA) prepared in PreparationExample 1 and 34.918 g (0.086 mol) of the diamine (DMICPD) prepared inPreparation Example 2 were completely dissolved in 322.091 g ofanhydrous N-methyl pyrrolidone (NMP).

Then, under an ice bath, 20.0 g (0.089 mol) of cyclohexane dianhydride(CHDA) was added to the solution and stirred at room temperature forabout 16 hours to prepare a copolymer P-7 for a liquid crystal alignmentagent. The molecular weight of the copolymer was confirmed by GPC, andas a result, the weight average molecular weight (Mw) was 23,000 g/mol.

Synthesis Example 8

9.605 g (0.048 mol) of the diamine (pIDA) prepared in PreparationExample 1 and 19.399 g (0.048 mol) of the diamine (DMICPD) prepared inPreparation Example 2 were completely dissolved in 277.692 g ofanhydrous N-methyl pyrrolidone (NMP).

Then, under an ice bath, 20.0 g (0.089 mol) of cyclohexane dianhydride(CHDA) was added to the solution and stirred at room temperature forabout 16 hours to prepare a copolymer P-8 for a liquid crystal alignmentagent. The molecular weight of the copolymer was confirmed by GPC, andas a result, the weight average molecular weight (Mw) was 25,000 g/mol.

Comparative Synthesis Example 1

10.374 g (0.096 mol) of p-phenylenediamine (p-PDA) was completelydissolved in 172.121 g of anhydrous N-methyl pyrrolidone (NMP).

Then, under an ice bath, 20.0 g (0.089 mol) of1,3-dimethylcyclobutane-1,2,3,4-tetracarboxylic acid dianhydride(DMCBDA) was added to the solution and stirred at room temperature forabout 16 hours to prepare a polymer R-1 for a liquid crystal alignmentagent. The molecular weight of the polymer was confirmed by GPC, and asa result, the weight average molecular weight (Mw) was 25,000 g/mol.

Comparative Synthesis Example 2

19.399 g (0.048 mol) of the diamine (DMICPD) prepared in Preparation

Example 2 and 9.605 g (0.048 mol) of 4,4′-oxydianiline (ODA) werecompletely dissolved in 277.689 g of anhydrous N-methyl pyrrolidone(NMP).

Then, under an ice bath, 20.0 g (0.089 mol) of1,3-dimethylcyclobutane-1,2,3,4-tetracarboxylic acid dianhydride(DMCBDA) was added to the solution and stirred at room temperature forabout 16 hours to prepare a polymer R-2 for a liquid crystal alignmentagent. The molecular weight of the copolymer was confirmed by GPC, andas a result, the weight average molecular weight (Mw) was 23,000 g/mol.

Comparative Synthesis Example 3

9.605 g (0.048 mol) of the diamine (pIDA) prepared in PreparationExample 1 and 5.187 g (0.048 mol) of p-phenylenediamine (p-PDA) werecompletely dissolved in 197.157 g of anhydrous N-methyl pyrrolidone(NMP).

Then, under an ice bath, 20.0 g (0.089 mol) of1,3-dimethylcyclobutane-1,2,3,4-tetracarboxylic acid dianhydride(DMCBDA) was added to the solution and stirred at room temperature forabout 16 hours to prepare a polymer R-3 for a liquid crystal alignmentagent. The molecular weight of the copolymer was confirmed by GPC, andas a result, the weight average molecular weight (Mw) was 23,500 g/mol.

Comparative Synthesis Example 4

A copolymer S-1 for a liquid crystal alignment agent was prepared in thesame manner as in Synthesis Example 1, except that6-(4-aminophenyl)pyridin-3-amine represented by the following ChemicalFormula A was used instead of the diamine (pIDA) compound 4 prepared inPreparation Example 1.

Comparative Synthesis Example 5

A copolymer S-2 for a liquid crystal alignment agent was prepared in thesame manner as in Synthesis Example 1, except that4,4′-diaminodiphenylamine represented by the following Chemical FormulaB was used instead of the diamine (pIDA) compound 4 prepared inPreparation Example 1.

Comparative Synthesis Example 6

A copolymer S-3 for a liquid crystal alignment agent was prepared in thesame manner as in Synthesis Example 1, except thatN-(2,6-bis(trifluroromethyl)-4-aminophenyI)-1,4-phenylenediaminerepresented by the following Chemical Formula C was used instead of thediamine (pIDA) compound 4 prepared in Preparation Example 1.

Examples 1 to 8 and Comparative Examples 1 to 6: Preparation of LiquidCrystal Alignment Agent Example 1

20 g of the copolymer (P-1) for a liquid crystal alignment agent ofSynthesis Example 1 was dissolved in a mixed solvent of 8.65 g of NMP,19.95 g of

-butyrolactone (GBL), and 11.4 g of 2-butoxyethanol to obtain a 5 wt %solution. Then, the solution thus obtained was subjected to pressurefiltration using a filter having a pore size of 0.1 μm and made ofpoly(tetrafluoroethylene) to prepare a liquid crystal alignment agentA-1.

Example 2

20 g of the copolymer (P-2) for a liquid crystal alignment agent ofSynthesis Example 2 was dissolved in a mixed solvent of 8.65 g of NMP,19.95 g of GBL, and 11.4 g of 2-butoxyethanol to obtain a 5 wt %solution. Then, the solution thus obtained was subjected to pressurefiltration using a filter having a pore size of 0.1 μm and made ofpoly(tetrafluoroethylene) to prepare a liquid crystal alignment agentA-2.

Example 3

20 g of the copolymer (P-3) for a liquid crystal alignment agent ofSynthesis Example 3 was dissolved in a mixed solvent of 8.65 g of NMP,19.95 g of GBL, and 11.4 g of 2-butoxyethanol to obtain a 5 wt %solution. Then, the solution thus obtained was subjected to pressurefiltration using a filter having a pore size of 0.1 μm and made ofpoly(tetrafluoroethylene) to prepare a liquid crystal alignment agentA-3.

Example 4

20 g of the copolymer (P-4) for a liquid crystal alignment agent ofSynthesis Example 4 was dissolved in a mixed solvent of 8.65 g of NMP,19.95 g of GBL, and 11.4 g of 2-butoxyethanol to obtain a 5 wt %solution. Then, the solution thus obtained was subjected to pressurefiltration using a filter having a pore size of 0.1 μm and made ofpoly(tetrafluoroethylene) to prepare a liquid crystal alignment agentA-4.

Example 5

20 g of the copolymer (P-5) for a liquid crystal alignment agent ofSynthesis Example 5 was dissolved in a mixed solvent of 8.65 g of NMP,19.95 g of GBL, and 11.4 g of 2-butoxyethanol to obtain a 5 wt %solution. Then, the solution thus obtained was subjected to pressurefiltration using a filter having a pore size of 0.1 μm and made ofpoly(tetrafluoroethylene) to prepare a liquid crystal alignment agentA-5.

Example 6

20 g of the copolymer (P-6) for a liquid crystal alignment agent ofSynthesis Example 6 was dissolved in a mixed solvent of 8.65 g of NMP,19.95 g of GBL, and 11.4 g of 2-butoxyethanol to obtain a 5 wt %solution. Then, the solution thus obtained was subjected to pressurefiltration using a filter having a pore size of 0.1 μm and made ofpoly(tetrafluoroethylene) to prepare a liquid crystal alignment agentA-6.

Example 7

20 g of the copolymer (P-7) for a liquid crystal alignment agent ofSynthesis Example 7 was dissolved in a mixed solvent of 8.65 g of NMP,19.95 g of GBL, and 11.4 g of 2-butoxyethanol to obtain a 5 wt %solution. Then, the solution thus obtained was subjected to pressurefiltration using a filter having a pore size of 0.1 μm and made ofpoly(tetrafluoroethylene) to prepare a liquid crystal alignment agentA-7.

Example 8

20 g of the copolymer (P-8) for a liquid crystal alignment agent ofSynthesis Example 8 was dissolved in a mixed solvent of 8.65 g of NMP,19.95 g of GBL, and 11.4 g of 2-butoxyethanol to obtain a 5 wt %solution. Then, the solution thus obtained was subjected to pressurefiltration using a filter having a pore size of 0.1 μm and made ofpoly(tetrafluoroethylene) to prepare a liquid crystal alignment agentA-3.

Comparative Example 1

20 g of the copolymer (R-1) for a liquid crystal alignment agent ofComparative Synthesis Example 1 was dissolved in a mixed solvent of 8.65g of NMP, 19.95 g of GBL, and 11.4 g of 2-butoxyethanol to obtain a 5 wt% solution. Then, the solution thus obtained was subjected to pressurefiltration using a filter having a pore size of 0.1 μm and made ofpoly(tetrafluoroethylene) to prepare a liquid crystal alignment agentR′-1.

Comparative Example 2

20 g of the copolymer (R-2) for a liquid crystal alignment agent ofComparative Synthesis Example 2 was dissolved in a mixed solvent of 8.65g of NMP, 19.95 g of GBL, and 11.4 g of 2-butoxyethanol to obtain a 5 wt% solution. Then, the solution thus obtained was subjected to pressurefiltration using a filter having a pore size of 0.1 μm and made ofpoly(tetrafluoroethylene) to prepare a liquid crystal alignment agentR′-2.

Comparative Example 3

20 g of the copolymer (R-3) for a liquid crystal alignment agent ofComparative Synthesis Example 3 was dissolved in a mixed solvent of 8.65g of NMP, 19.95 g of GBL, and 11.4 g of 2-butoxyethanol to obtain a 5 wt% solution. Then, the solution thus obtained was subjected to pressurefiltration using a filter having a pore size of 0.1 μm and made ofpoly(tetrafluoroethylene) to prepare a liquid crystal alignment agentR′-3.

Comparative Example 4

20 g of the copolymer (S-1) for a liquid crystal alignment agent ofComparative Synthesis Example 4 was dissolved in a mixed solvent of 8.65g of NMP, 19.95 g of GBL, and 11.4 g of 2-butoxyethanol to obtain a 5 wt% solution. Then, the solution thus obtained was subjected to pressurefiltration using a filter having a pore size of 0.1 μm and made ofpoly(tetrafluoroethylene) to prepare a liquid crystal alignment agentS′-1.

Comparative Example 5

20 g of the copolymer (S-2) for a liquid crystal alignment agent ofComparative Synthesis Example 5 was dissolved in a mixed solvent of 8.65g of NMP, 19.95 g of GBL, and 11.4 g of 2-butoxyethanol to obtain a 5 wt% solution. Then, the solution thus obtained was subjected to pressurefiltration using a filter having a pore size of 0.1 μm and made ofpoly(tetrafluoroethylene) to prepare a liquid crystal alignment agentS′-2.

Comparative Example 6

20 g of the copolymer (S-3) for a liquid crystal alignment agent ofComparative Synthesis Example 6 was dissolved in a mixed solvent of 8.65g of NMP, 19.95 g of GBL, and 11.4 g of 2-butoxyethanol to obtain a 5 wt% solution. Then, the solution thus obtained was subjected to pressurefiltration using a filter having a pore size of 0.1 μm and made ofpoly(tetrafluoroethylene) to prepare a liquid crystal alignment agentS′-3.

Experimental Example: Measurement of Physical Properties of LiquidCrystal Alignment Agent

1) Preparation of Liquid Crystal Alignment Cell

Each of the liquid crystal alignment agents obtained in Examples 1 to 8and Comparative Examples 1 to 6 was used to prepare a liquid crystalalignment cell.

Specifically, the liquid crystal alignment agent was coated onto theupper and lower substrates for a voltage holding ratio (VHR) in whichITO electrodes with a thickness of 60 nm and an area of 1 cm×1 cm werepatterned on a square glass substrate with a size of 2.5 cm×2.7 cm by aspin coating method, respectively. Then, the substrates coated with theliquid crystal alignment agent were placed on a hot plate at about 70°C. and dried for 3 minutes to evaporate the solvent. For alignmenttreatment of the coated substrates thus obtained, each of upper andlower coated substrates was irradiated with UV of 254 nm using anexposure equipped with a line polarizer. Thereafter, thealignment-treated upper and lower substrates were baked (cured) in anoven at about 230° C. for about 30 minutes to obtain a coating film witha thickness of 0.1 μm. Thereafter, a sealing agent impregnated with ballspacers with a size of 4.5 μm was coated onto the edges of the uppersubstrate excluding a liquid crystal inlet. The alignment films formedon the upper and lower substrates were then aligned such that they facedeach other and the alignment directions were aligned with each other,and the upper and lower substrates were bonded together and the sealingagent was cured with UV and heat to prepare an empty cell. Then, aliquid crystal was injected into the empty cells, and the inlet wassealed with a sealing agent to prepare a liquid crystal alignment cell.

2) Measurement of Voltage Holding Ratio (VHR)

The voltage holding ratio (VHR) which is an electrical property of theliquid crystal alignment cell thus prepared was measured using 6254 Cequipment manufactured by TOYO Corporation. The voltage holding ratio(VHR) was measured at 1 Hz and 60° C. (VHR 60 degrees 1 Hz n-LCconditions). The results of measuring the voltage holding ratios (VHR)of the liquid crystal alignment cells are shown in the following Table1.

3) Evaluation of Liquid Crystal Alignment Characteristics (AlternatingCurrent (AC) Afterimage)

Polarizing plates were attached to the upper and lower substrate platesof the above prepared liquid crystal alignment cell so as to beperpendicular to each other. The polarizing plate-attached liquidcrystal alignment cell was attached on a backlight having luminance of7000 cd/cm², and the luminance in a black state was measured using aluminance measuring instrument PR-880. Then, the liquid crystal cell wasoperated at room temperature with an alternating voltage of 5 V for 24hours. Thereafter, in the voltage-off state of the liquid crystal cell,luminance in the black state was measured as described above. Adifference between the initial luminance (L₀) measured before operationof the liquid crystal cell and the later luminance (L₁) measured afteroperation was divided by the initial luminance (L₀), and then multipliedby 100 to calculate a luminance fluctuation rate. When the calculatedluminance fluctuation rate is close to 0%, it means that the alignmentstability is excellent.

Through the measurement results of the luminance fluctuation rate, theafterimage level was evaluated under the following criteria. It ispreferable to minimize an AC afterimage. According to the measurementresults, when the luminance fluctuation rate was less than 10%, it wasevaluated as “excellent”, when the luminance fluctuation rate was 10% to20%, it was evaluated as “ordinary”, and when the luminance fluctuationrate was more than 20%, it was evaluated as “poor”. The results areshown in Table 1 below.

TABLE 1 Evaluation of physical property of liquid crystal liquid Firstdiamine Second diamine Dianhydride cell * crystal Content ContentContent AC VHR agent Type (mmol) Type (mmol) Type (mmol) afterimage (%)Example 1 A-1 PIDA 10 DMICPD 86 DMCBDA 89 Excellent 76 Example 2 A-2PIDA 36 DMICPD 36 DMCBDA 67 Excellent 75 Example 3 A-3 PIDA 10 DMICPD 89PMDA 92 Excellent 70 Example 4 A-4 PIDA 37 DMICPD 37 PDMA 69 Excellent71 Example 5 A-5 PIDA 7 DMICPD 66 BPDA 68 Excellent 70 Example 6 A-6PIDA 37 DMICPD 37 BPDA 68 Excellent 82 Example 7 A-7 PIDA 10 DMICPD 86CHDA 89 Excellent 71 Example 8 A-8 PIDA 48 DMICPD 48 CHDA 89 Excellent74 Comparative R′-1 PDA 96 — — DMCBDA 89 Poor 37 Example 1 ComparativeR′-2 ODA 48 DMICPD 48 DMCBDA 89 Excellent 35 Example 2 Comparative R′-3PIDA 48 PDA 48 DMCBDA 89 Poor 70 Example3 Comparative S′-1 Chemical 10DMICPD 86 DMCBDA 89 Ordinary 61 Example 4 Formula A Comparative S′-2Chemical 10 DMICPD 86 DMCBDA 89 Ordinary 70 Example 5 Formula BComparative S′-3 Chemical 10 DMICPD 86 DMCBDA 89 Poor 33 Example 6Formula C * Measured under an exposure dose of 0.1 to 0.5 J/cm²

As shown in Table 1, it was confirmed that since each of the liquidcrystal alignment agents of Examples 1 to 8 includes the copolymerproduced from the reaction product containing the imide-containing firstdiamine having a specific structure along with the second diamine havinga specific asymmetric structure, an excellent alignment property may beobtained without an initial thermosetting process, the voltage holdingratio (VHR) may be improved to as high as about 70% to about 82%, andthe AC afterimage may be maintained at an equivalent level or more.Particularly, the liquid crystal alignment agents of Examples 1 to 8 mayexhibit an excellent coating property to implement a high imidizationrate while having excellent processing properties, and may also exhibitexcellent effects in terms of electrical properties such as voltageholding ratio and direct current (DC) afterimage generated by the directcurrent/alternating voltage.

In contrast, in the case of the liquid crystal alignment agents ofComparative Examples 1 to 6, none of the imide-containing diamine havinga specific structure and the diamine having a specific asymmetricstructure were included or one of them was included in the reactionproduct during the preparation of the polymer, and as a result,electrical properties or alignment properties of the liquid crystalcells were remarkably deteriorated.

In particular, in the case of Comparative Example 1, the polymerincluding only para-phenylenediamine (p-PDA) as the diamine componentwas used, and as a result, it showed a remarkably reduced voltageholding ratio of about 37% and also showed a luminance fluctuation rateof more than 20%, and was thus evaluated as ‘poor’ in the AC afterimagetest. In the case of Comparative Example 2, of the copolymers, diamine(DMICPD) having a specific asymmetric structure was used, but4,4′-oxydianiline (ODA) was used instead of the imide-containing firstdiamine. As a result, there was no problem in the liquid crystalalignment properties, but it showed a remarkably reduced voltage holdingratio of about 35%. In the case of Comparative Example 3, of thecopolymers, the imide-containing diamine (PIDA) having a specificstructure was used, but para-phenylenediamine (p-PDA) was used insteadof the second diamine having a specific asymmetric structure. As aresult, the voltage holding ratio (VHR) may be maintained at theequivalent level or more, but there was a problem that a luminancefluctuation rate of more than 20% was observed, and thus was evaluatedas ‘poor’ in the AC afterimage test.

The invention claimed is:
 1. A copolymer for a liquid crystal alignmentagent comprising: one or more repeating units selected from the group ofa repeating unit represented by Chemical Formula 1, a repeating unitrepresented by Chemical Formula 2, and a repeating unit represented byChemical Formula 3; and one or more repeating units selected from thegroup of a repeating unit represented by Chemical Formula 4, a repeatingunit represented by Chemical Formula 5, and a repeating unit representedby Chemical Formula 6, wherein a molar ratio between the one or morerepeating units selected from the group of the repeating unitrepresented by Chemical Formula 1, the repeating unit represented byChemical Formula 2, and the repeating unit represented by ChemicalFormula 3, and the one or more repeating units selected from the groupof the repeating unit represented by Chemical Formula 4, the repeatingunit represented by Chemical Formula 5, and the repeating unitrepresented by Chemical Formula 6 is 40:60 to 95:5:

wherein, in the Chemical Formulae 1 to 6, R¹, R², R³, and R⁴ are eachindependently hydrogen or a C₁₋₁₀ alkyl, provided that R¹ and R² are notboth hydrogen, and that R³ and R⁴ are not both hydrogen, X¹, X³, and X⁵are each independently a tetravalent organic group represented byChemical Formula 7:

wherein, in the Chemical Formula 7, R⁵, R⁶, R⁷, and R⁸ are eachindependently hydrogen or a C₁₋₆ alkyl, X², X⁴, X⁶, X⁷, X⁸, and X⁹ areeach independently a tetravalent organic group, wherein in thetetravalent organic group represented by X², X⁴, X⁶, X⁷, X⁸, and X⁹, oneor more of H is optionally substituted with a halogen or one or more of—CH₂— is optionally substituted with —O—, —CO—, —S—, —SO—, —SO₂—, or—CONH—, in the Chemical Formulae 1 to 3, Y¹, Y², Y³, Y⁴, Y⁵, and Y⁶ areeach independently a divalent organic group represented by ChemicalFormula 8:

wherein, in the Chemical Formula 8, R⁹ and R¹⁰ are each independently ahalogen, a cyano, a C₁₋₁₀ alkyl, a C₂₋₁₀ alkenyl, a C₁₋₁₀ alkoxy, aC₁₋₁₀ fluoroalkyl, or a C₁₋₁₀ fluoroalkoxy, p and q are eachindependently an integer of 0 to 4, L¹ is a single bond, —O—, —CO—, —S—,—SO₂—, —C(CH₃)₂—, —C(CF₃)₂—, —CONH—, —COO—, —(CH₂)_(z)—, —O(CH₂)_(z)O—,—O(CH₂)_(z)—, —OCH₂—C(CH₃)₂—CH₂O—, —COO—(CH₂)_(z)—OCO—, or—OCO—(CH₂)_(z)—COO—, wherein each z is independently an integer of 1 to10, k and m are each independently an integer of 0 to 3, and n is aninteger of 0 to 3, wherein when n=0, k is 1 to 3, and when k=0, m and nare each independently 1 to 3, in Chemical Formulae 4 to 6, Z¹, Z², andZ³ are each independently a divalent organic group represented byChemical Formula 9:

wherein, in the Chemical Formula 9, A¹ is an element of Group 15selected from the group of nitrogen, phosphorus, arsenic, antimony orbismuth, R¹¹ is hydrogen or a C₁₋₁₀ alkyl, a is an integer of 1 to 3,and at least one of A² and A⁵ is nitrogen and the other is carbon, andA³ and A⁴ are carbon.
 2. The copolymer for a liquid crystal alignmentagent according to claim 1, wherein X², X⁴, X⁶, X⁷, X⁸, and X⁹ eachindependently include a tetravalent organic group represented byChemical Formula 10:

wherein, in the Chemical Formula 10, R⁵, R⁶, R⁷, and R⁸ are eachindependently hydrogen or a C₁₋₆ alkyl, R¹² and R¹³ are eachindependently hydrogen or a C₁₋₁₀ alkyl, L² is any one selected from thegroup consisting of a single bond, —O—, —CO—, —S—, —SO—, —SO₂—,—CR¹⁴R¹⁵—, —CONH—, —COO—, —(CH₂)_(b)—, —O(CH₂)_(b)O—,—COO—(CH₂)_(b)—OCO—, —HN—(CH₂)_(b)—NH—, —R¹⁴N—(CH₂)_(b)—NR¹⁵—,phenylene, and combinations thereof, wherein R¹⁴ and R¹⁵ are eachindependently hydrogen, a C₁₋₁₀ alkyl, or a C₁₋₁₀ fluoroalkyl, and eachb is independently an integer of 1 to
 10. 3. The copolymer for a liquidcrystal alignment agent according to claim 1, wherein the ChemicalFormula 8 is a divalent organic group represented by Chemical Formula 11or Chemical Formula 12:

wherein, in the Chemical Formula 12, L³ is a single bond, —O—, —SO₂—, or—CR¹⁶R¹⁷—, wherein R¹⁶ and R¹⁷ are each independently hydrogen or aC₁₋₁₀ alkyl.
 4. The copolymer for a liquid crystal alignment agentaccording to claim 1, wherein in the Chemical Formula 9, A¹ is nitrogen,R¹¹ is hydrogen, and a is
 1. 5. The copolymer for a liquid crystalalignment agent according to claim 1, wherein Chemical Formula 9includes one or more repeating units selected from the group of ChemicalFormula 9-1, Chemical Formula 9-2, and Chemical Formula 9-3:

wherein, in the Chemical Formulae 9-1 to 9-3, A¹ to A⁵, R¹¹, and a arethe same as defined in claim
 1. 6. The copolymer for a liquid crystalalignment agent according to claim 1, comprising one or more repeatingunits selected from the group consisting of repeating units representedby Chemical Formulae 13 to 21:

wherein, in the Chemical Formulae 13 to 21, R¹ to R⁴, X¹ to X⁹, Y¹ toY⁶, and Z¹ to Z³ are the same as defined in claim 1, and m¹ to m¹⁸ areeach independently an integer of 1 to
 500. 7. The copolymer for a liquidcrystal alignment agent according to claim 1, wherein the copolymer fora liquid crystal alignment agent has a weight average molecular weightof 1000 g/mol to 200,000 g/mol.
 8. A liquid crystal alignment agentcomprising the copolymer for a liquid crystal alignment agent accordingto claim
 1. 9. A method of producing a liquid crystal alignment film,the method comprising the steps of: coating the liquid crystal alignmentagent of claim 8 onto a substrate to form a coating film; drying thecoating film; irradiating the coating film with light or rubbing thecoating film immediately after the drying step to perform an alignmenttreatment; and heat-treating and curing the alignment-treated coatingfilm.
 10. The method of producing a liquid crystal alignment filmaccording to claim 9, wherein the liquid crystal alignment agent isdissolved or dispersed in an organic solvent.
 11. The method ofproducing a liquid crystal alignment film according to claim 9, whereinthe step of drying the coating film is performed at 50° C. to 150° C.12. The method of producing a liquid crystal alignment film according toclaim 9, wherein in the alignment treatment step, the light irradiationis performed by irradiating polarized ultraviolet rays having awavelength of 150 nm to 450 nm.
 13. The method of producing a liquidcrystal alignment film according to claim 9, wherein in the step ofheat-treating and curing the coating film, the heat treatmenttemperature is 150° C. to 300° C.
 14. A liquid crystal alignment filmproduced by the method of claim
 9. 15. A liquid crystal display devicecomprising the liquid crystal alignment film according to claim 14.